Numerous opportunities exist in which accurate measurements of volume and area are essential, particularly of objects or portions of objects having variable volumes or areas. For example, in the field of medicine, the recording of breathing volumes in patients is often quite critical. Unfortunately these measurements are often crude and inaccurate, or at best rely upon outmoded technical modalities or upon unreliable. The recording of breathing volumes in patients is currently either performed by connecting a volume flow-sensing device to a subject's airway (e.g. by use of a spirometer or tachymeter) or by measuring the mechanical excursions of the chest and abdominal walls. For long-term monitoring purposes, the airway-based techniques are inappropriate since they interfere with normal breathing and are unpleasant for the patient. This is particularly relevant to children and certain other patients. Although airway-based techniques are currently used in patients dependent on respiratory-assist devices there may be less intrusive and more reliable means of obtaining such data.
Similarly, techniques that rely on recordings of chest and abdominal wall movements are either strain gauge based (recording of changes in body circumference length), or based on elastic inductive electrical conductor loops arranged around the chest and abdomen of the patient. Recordings of the inductance of the loops can then be used to estimate the magnitude of cross sectional area variations of the chest and abdominal compartments. U.S. Pat. No. 4,308,872 is an example of this self-inductance loop estimation technology. Such methods might be used for quantitative measurements of respiratory volumes only after a calibration procedure where the patient breathes known air volumes with variable respiration movement distributions between the chest and abdominal compartments.
Currently, most devices for measurements of chest volume variations have shortcomings related to calibration, stability, accuracy or reliability. The methods are either based on measurements of circumference of the chest and abdomen (strain-gauge transducers), or on measurements of the electrical inductance of conductor loops arranged around the chest and abdomen. The reason for using abdominal sensors is that the downward movement of the diaphragm during inspiration causes volume changes of both the chest and abdomen that have to be added to estimate the lung volume excursions. There is no fixed ratio between the abdominal and chest volume variations. Indeed, the relative contributions to total volume variations might even vary as a consequence of respiratory effort, airway resistance or sleep state. Thus, independent calibrations of both measurement sites are necessary (when using known techniques) to estimate the actual volume variations caused by respiration.
The strain gauge or circumferential distance methods have no simple or reproducible relation between the measured variations and the volumes that are measured. This relation depends on assumptions about the relation between the area enclosed by the loop and the length of the loop that are valid only for a fixed geometry. Although some of the methods based on inductance may claim that area is measured (i.e., it is assumed to be proportional to loop inductance), the assumption is only valid as long as the relative shape of the loop is conserved. Unfortunately, this is not the case for the cross-sectional area variations of the human chest or abdomen that are caused by respiration.
Common to all area-based methods is a considerable uncertainty in the calculations of volume variations from the estimated area variations. One reason for this is that only point samples of the area (at the sites of the transducers) are measured. To relate area variations to volumes, some means of calibration is necessary. The calibration procedure depends highly upon patient cooperation to obtain coefficients for both chest and abdominal measurement sites, and is virtually inapplicable to small children, patients with dyspnea (shortness of breath), and unconscious patients.
This invention describes novel systems and methods for volume and area measurements, based on electrical inductance, in ways which eliminate the shortcomings discussed above. The invention does not depend on patient cooperation for calibration and is a true volume or area measurement method that does not depend on assumptions about the relations between circumference, area and volume.